A heat exchanger of the above kind is described in EP-A-0 431 917. This known heat exchanger serves as a radiator for cooling the engine of a motor vehicle, and also as the condenser of an air conditioning system for the cabin of the vehicle. The first fluid in that case is atmospheric air, while the second fluid is engine coolant and the third fluid is the refrigerant of the air conditioning system. The two pockets disposed between two consecutive gaps are juxtaposed to each other, and each of these pockets extends over a fraction of the surface area of the stack. They are spaced away from each other by a certain amount, and each pocket lies adjacent to two consecutive gaps. The total size of that heat exchanger, in the direction in which the pockets are juxtaposed (i.e. the stacking direction), corresponds to the sum of the dimensions in that direction of an equivalent separate radiator and separate condenser, augmented by an additional amount corresponding to the sum of the distances between the pairs of juxtaposed pockets.
In air conditioning systems for vehicle cabins, it is current practice to arrange, in series in the flow path of air to be treated, an evaporator which is part of the refrigerant fluid circuit and which is arranged to receive this air, together with a radiator for heating the air. Since very little space is generally available for air conditioning systems, there is nowadays a tendency, in order to save space, to replace these two heat exchangers with a single combined heat exchanger which performs both functions. However, such a space saving cannot be achieved with the arrangement disclosed by the prior document cited above, for the reasons already explained.